<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 140: map&lt;Key, T&gt;::value_type does not satisfy the assignable requirement</title>
<meta property="og:title" content="Issue 140: map&lt;Key, T&gt;::value_type does not satisfy the assignable requirement">
<meta property="og:description" content="C++ library issue. Status: NAD Editorial">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue140.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#NAD_Editorial">NAD Editorial</a> status.</em></p>
<h3 id="140"><a href="lwg-closed.html#140">140</a>. map&lt;Key, T&gt;::value_type does not satisfy the assignable requirement</h3>
<p><b>Section:</b> 23.4.3 <a href="https://wg21.link/map">[map]</a> <b>Status:</b> <a href="lwg-active.html#NAD_Editorial">NAD Editorial</a>
 <b>Submitter:</b> Mark Mitchell <b>Opened:</b> 1999-04-14 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#map">issues</a> in [map].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Editorial">NAD Editorial</a> status.</p>
<p><b>Discussion:</b></p>
<blockquote>
  <p>23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a><br/>
  <br/>
  expression&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return type
  &nbsp;&nbsp;&nbsp;&nbsp; pre/post-condition<br/>
  -------------&nbsp;&nbsp;&nbsp;&nbsp; ----------- &nbsp;&nbsp;&nbsp;&nbsp;
  -------------------<br/>
  X::value_type&nbsp;&nbsp;&nbsp; T
  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  T is assignable<br/>
  <br/>
  23.4.3 <a href="https://wg21.link/map">[map]</a><br/>
  <br/>
  A map satisfies all the requirements of a container.<br/>
  <br/>
  For a map&lt;Key, T&gt; ... the value_type is pair&lt;const Key, T&gt;.</p>
</blockquote>

<p>There's a contradiction here. In particular, `pair&lt;const Key,
T&gt;' is not assignable; the `const Key' cannot be assigned
to. So,&nbsp; map&lt;Key, T&gt;::value_type does not satisfy the
assignable requirement imposed by a container.</p>

<p><i>[See issue <a href="lwg-defects.html#103" title="set::iterator is required to be modifiable, but this allows modification of keys (Status: CD1)">103</a><sup><a href="https://cplusplus.github.io/LWG/issue103" title="Latest snapshot">(i)</a></sup> for the slightly related issue of
modification of set keys.]</i></p>



<p><b>Rationale:</b></p>
<p>The LWG believes that the standard is inconsistent, but that this
is a design problem rather than a strict defect. May wish to
reconsider for the next standard.</p>




</body>
</html>
